I have never heard Bill Cooper ever stated he believed in Extra Terrestrial and has spoken against it https://youtu.be/45aeUdq9Zj0. He has mentioned aliens but that is not the same as ET's. This may be based on the many elongated & enlarged skulls found all over the planet and found in tomes.

Whether these are still alive is open for debate but considering many people are ignorant of the presence of such skulls from our history and currently being removed from museums; this could indicate they do. The mathematical word we live in and the secret societies love of geometry, symbols, rituals etc also could support this theory.